This invention relates to center fed multiarm spiral antennas that are configured for transmission and reception of either left- or right-hand circularly polarized electromagnetic energy.
As is known in the art, a center fed, multiarm spiral antenna having N-arms exhibits N-1 independent operating modes wherein the individual operating modes are determined by the phase difference between the currents induced in the antenna arms. In particular, a first operational mode (commonly referred to as the sum or .SIGMA. mode and identified herein as the M=1 mode) is attained when the phase difference between the excitation currents in adjacent antenna arms is 2.pi./N radians. This mode of operation produces a circularly polarized, symmetrical, single lobed radiation pattern that exhibits maximum field strength along the antenna boresight axis. Higher order modes (i.e., M=2, 3 . . . , [N-1]), often called difference (or .DELTA.) modes, are attained when the phase difference between the current in adjacent antenna arms is 2 .pi. M/N radians. Each of these higher order modes is characterized by a radiation pattern that exhibits a null along the antenna boresight axis and maximum field strength along a cone of revolution about the boresight axis, with the cone angle increasing and the relative peak field strength decreasing as the mode number increases.
As is also known in the art, each operating mode, M, of a center fed multiarm spiral antenna exhibits a circular radiation zone that is m.lambda./.pi. in diameter, where .lambda. is the freespace wavelength of the antenna operating frequency. These modes exhibit right-hand circular polarization when the antenna is wound in the counterclockwise direction and exhibit left-hand circular polarization when the antenna is wound in the clockwise direction.
To obtain simultaneous right-hand and left-hand circular polarization with a center fed multiarm spiral antenna, a technique known as "mode conversion" is utilized wherein the antenna is configured such that the electrical length of the antenna arms and hence the effective antenna radius is less than that required to emit radiation at one or more of the higher operating modes. In such an arrangement, excitation currents that would normally result in an operating mode that is higher than the modes that can be supported by the electrical radius of the antenna are reflected and flow inwardly toward the center of the antenna. When the reflected, inwardly flowing currents of the adjacent antenna arms reach an in-phase condition, the antenna radiates circularly polarized electromagnetic energy that exhibits an equivalent (or converted) mode order of N-M, where M is the original or normal mode number. As previously stated, the polarization sense of each converted mode is opposite to the polarization sense that would normally be induced in the antenna: Thus, both right- and left-hand circular polarization are simultaneously exhibited by a single spiral antenna. For example, a six-element logarithmic (equal angle) spiral antenna that is wound in the counterclockwise direction and configured to exhibit an equivalent electrical circumference of 3 .lambda. will exhibit right-hand circular polarization for mode numbers 1 and 2 (phase difference of .pi./3 radians and 2.pi./3 radians respectively, at the antenna feed points). However, when excitation currents that would normally result in operating modes 4 and 5 are induced (phase difference of 4.pi./3 radians and 5.pi./3 radians, respectively, at the antenna feedpoints), the antenna will exhibit left-hand circular polarization at mode numbers 2 and 1 by virtue of the mode conversion process.
As is known in the art, and as is demonstrated by Kuo et al., U.S. Pat. No. 3,562,756, converted mode spiral antennas that operate over a relatively narrow frequency range can be realized by suitably establishing the physical length of the antenna arms so that reflection occurs at the physical termination of each antenna arm. As is disclosed, for example, in Ingerson, U.S. Pat. No. 3,681,772 and Lamberty et al., U.S. Pat. No. 4,243,993, converted mode operation can be attained over a relatively wide frequency range by controlling the effective electrical length of each antenna arm rather than by physically terminating the antenna arms. In effect, such antennas are configured so that the electrical length of each antenna arm is inversely proportional to the frequency of the excitation signal. In the ideal case, such a configuration thus provides an antenna having a constant electrical radius relative to the wavelength of signals within the antenna bandwidth.
In the arrangement disclosed in the above-referenced patent to Ingerson, which is identified as a modulated arm width (MAW) spiral antenna, each antenna arm is formed by a series of "cells" with each cell being a section of antenna arm that includes a first, relatively narrow width dimension followed by a second section of antenna arm of substantially greater width dimension. These cells or "modulations" are positioned along the antenna arms to establish impedance discontinuities or reflection regions (denoted as "stop bands" in the Ingerson patent) which are intended to selectively reflect the outwardly flowing currents. In the arrangement disclosed by Ingerson, outwardly flowing currents are reflected when the length of a cell corresponds to .lambda./2. Thus, in concept, a relatively constant electrical radius can be obtained by utilizing a plurality of modulations in each antenna arm with cell length increasing as a function of the distance between the center of the antenna and the location of a particular cell. By also establishing the position of the arm width modulations (cells) so that currents produced by higher modes of excitation are reflected, whereas the lower modes produce radiation in the conventional manner, operation is achieved with both left-hand and right-hand circular polarization.
In the spiral antenna disclosed in the previously referenced patent to Lamberty et al., the cells are configured to form choke (reactive) elements which cause each cell to resonate at predetermined frequencies. In effect, each antenna arm of the apparatus disclosed by Lamberty et al. can be considered to be a series of cascaded, parallel resonant circuits that are interconnected with transmission lines wherein each successive resonant circuit exhibits a somewhat lower resonant frequency and the length of the interconnecting transmission lines increase with respect to each successive pair of resonant circuits. In the spiral antenna configuration disclosed in the patent to Lamberty et al., appropriately positioning the choke elements and suitably establishing the distance therebetween results in converted mode operation with the antenna exhibiting a relatively constant electrical radius.
Although spiral antennas of the type disclosed by Ingerson and Lamberty et al. are satisfactory in some situations, both of these arrangements exhibit some disadvantages and drawbacks. In particular, neither the abrupt impedance transitions of antennas configured in accordance with the teaching of Ingerson nor the resonant cell structure of spiral antennas constructed in accordance with the teaching of Lamberty et al., totally reflect outwardly flowing antenna excitation currents. The residual excitation current that is not reflected and continues to flow outwardly along the spiral antenna arms not only reduces the relative field strength of the converted mode radiation that is induced by the reflected energy, but also results in other undesired effects. In this regard, the cells of an antenna configured in accordance with the teaching of the Ingerson and Lamberty et al. patents contain abrupt arm width transitions and thus nearly frequency independent impedance discontinuities. These discontinuities reflect signals at the cell design frequency and at integer multiples thereof. Thus, excitation current that passes beyond the desired electrical radius of the antenna (i.e., current that is not reflected from the cell of length .lambda./2 in the arrangement disclosed by Ingerson and the cell that exhibits fundamental resonance at the excitation frequency in the apparatus disclosed by Lamberty et al.), can be reflected both by the physical terminus of the antenna arm and by the impedance discontinuities of the outwardly located cells that are designed to reflect energy at a different (lower) frequency. Any such additional reflection causes additional undesired radiation.
Futher, some difficulty can be encountered in fabricating a spiral antenna of the type disclosed in the Lamberty et al. patent for use at higher microwave frequencies. In this regard, realization of a high Q (quality factor) for the high frequency reactive cells requires that the width of conductors with the cell (and the spacing therebetween) be closely controlled. When the required dimensional constraints are not fully met, cell signal reflection decreases below the design value and an undesired amount of the excitation current passes outwardly beyond the desired reflection point.
The failure of the prior art antennas to act as ideal constant electrical radius antennas and the attendant undesired radiation can cause asymmetry of the radiation patterns relative the antenna boresight axis. Moreover, because of the undesired radiation, the characteristics of a prior art spiral antenna are to some degree both frequency and polarization dependent. Although this nonideal performance may not substantially affect performance of some systems that use a spiral antenna, substantial compromises in system performance and/or system complexity can result in certain systems that require highly symmetrical radiation patterns and uniform frequency characteristics. For example, amplitude monopulse tracking systems or angle-of-arrival systems that ideally are independent of received signal polarization and continuously operable over a multi-octave frequency band require an antenna having radiation patterns that are highly symmetric about the antenna axis and are substantially independent of both frequency and polarization sense.